This invention relates to a self-diagnosing method for diagnosing a selective catalytic reduction (SCR) system of a vehicle, which comprises an internal combustion engine, wherein said SCR system is arranged downstream of the engine, wherein the SCR system comprises at least one exhaust gas sensor being sensitive to nitrogen oxides (NOx) gas.
The invention also relates to a computer program comprising program code means for performing the inventive method, a computer program product comprising program code means stored on a computer readable medium for performing the inventive method, and a computer system for implementing, the inventive method.
The method of the invention is particularly applicable for diagnosing a SCR system of large compression ignition engine that is configured to be powered with diesel or similar fuel types. Such an engine is frequently used in for example heavy duty trucks, buses, wheel loaders, articulated haulers, marine applications, or the like. The invention is however useful also for vehicles in the medium and light duty segments.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has a high efficiency of and is one of the best converters of fossil energy. NOx emission concentration is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (airfuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off.
Reducing the oxides of nitrogen (NO and NO2, referred to as NOx) and particulate matter (PM) in exhaust gases from a diesel engine has become a very important problem in view of the protection of environment and the saving of finite fossil energy supply.
Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst are known to achieve high NOx conversion efficiency. Urea based SCR catalysts use gaseous ammonia as the active NOx reducing reductant. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (NHCO) and gaseous ammonia (NH3), which is then used to convert NOx.
However, in such systems, urea injection levels have to be very precisely controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip. In a typical urea-based SCR catalyst system, the amount of urea injected is in proportion to the exhaust gas NOx concentration that represents a trade-off between maximum NOX conversion and minimum ammonia slip.
The NOx conversion efficiency of a SCR system can normally be continuously derived during operation of the vehicle, for example by means of at least one NOx sensor arranged downstream of a SCR catalyst. The engine management system may set different diagnostic troubles codes if registered values are outside predetermined ranges, such as low NOx reduction level in SCR catalyst, NOx sensor error indication, low exhaust gas heating performance, high engine NOx output level, or the like. When a vehicle with one or more of such general diagnostic trouble codes is repaired, it may be difficult to exactly determine the root cause behind the trouble code or irrational NOx values, because there are many different alternative causes that may result in the same diagnose.
Furthermore, after replacing parts of the SCR system, the testing and verification procedure of the correct function of the SCR system is time consuming, and unreliable. Document US2011061372A for example shows an on board exhaust diagnostic system including a SCR efficiency testing sequence that is performed during driving of the vehicle. For this reason, the testing and verification procedure is sometimes completely omitted, thereby risking delivery of a repaired vehicle that does not operate correctly.
There is thus a need for an improved method for diagnosing a SCR system removing the above mentioned disadvantages.
It is desirable to provide an inventive self-diagnosing method for diagnosing a SCR system of a vehicle, where the previously mentioned problem is at least partly avoided.
The vehicle according to the inventive method comprises an internal combustion engine and the SCR system is arranged downstream of the engine. The SCR system comprises at least one exhaust gas sensor being sensitive to nitrogen oxides (NOx) gas. The method comprises a first diagnosing sequence of:                ensuring that said vehicle is in a stationary state;        controlling said engine to operate in a high NOx output engine operating state and in a low NOx output engine operating state;        registering an output signal of said at least one exhaust gas sensor when the engine operates in each of said high NOx output engine operating state and low NOx output engine operating state; and        diagnosing NOx measurement performance of said at least one exhaust gas sensor on the basis of said registered sensor output.        
The inventive method is a self-diagnosing method comprising at least one predefined test sequence. There is thus no need for a technician himself to develop and realise any test sequences of the engine and/or SCR system, and to draw any potentially erroneous subjective conclusions therefrom. The inventive method also reduces the likelihood that untested repaired vehicles are being delivered back to the customer, thereby reducing the risk of return repairs and customer dissatisfaction. Instead a predetermined self-diagnosing method is automatically conducted upon command of the technician. The self-diagnosing method further includes predetermined threshold values for evaluating the performance of the SCR system, such as for example the performance of one or more exhaust gas sensors. A predetermined threshold value may herein be represented by a predetermined quantified value, or calculated using predetermined mathematical functions. This procedure makes the diagnosing result much more objective, reliable, and less time consuming. The at least one test sequence of the self-diagnosing method may be determined by engineers that are experts in SCR systems and exhaust gas aftertreatment systems, such that the result of the test can deliver an accurate and representative diagnose of the SCR system.
According to a preferred embodiment, the engine is controlled to operate alternating in a high NOx output engine operating state and in a low NOx output engine operating state. This alternating operation, which includes at least one transition between a high NOx output engine operating state and a low NOx output engine operating state, allows evaluation and diagnose of the NOx measurement performance of the exhaust gas sensor based on registered sensor output. The measurement performance is here evaluated in terms of the exhaust gas sensor's capability of accurately measuring high and low NOx levels. If for example the engine operates in a high NOx state and subsequently in a low NOx state, the difference in measured NOx level as provided by the exhaust gas sensor can be compared with a predetermined value, and based on this comparison a conclusion can be derived concerning the function of the exhaust gas sensor.
The fact that the vehicle is in a stationary state when the method is performed makes the test less time consuming and more accurate. There is also no longer a need to drive a fully loaded vehicle during the test, and the risk that the driver and diagnosing system do not operate consistently resulting in reduced quality and reliability of the diagnose result, is eliminated. Required exhaust gas temperature may be accomplished by means of applying vehicle internal engine loads, such as exhaust gas braking, aftertreatment hydrocarbon injection, high engine speed, or the like.
Further advantages are achieved by implementing one or several of the features of the dependent claims.
The first diagnosing sequence may further comprise controlling said engine to perform at least one transition from said high NOx output engine operating state to said low NOx output engine operating state, and from said low NOx output engine operating state to said high NOx output engine operating state, as well as registering said output signal from said at least one exhaust gas sensor at least before and after each of said transitions. By including at least two transitions, one from a high NOx state to a low NOx state, and oppositely, further conclusions can be derived about the functioning and quality of the exhaust gas sensor, thereby delivering an improved diagnosis.
The method may further comprise a second diagnosing sequence of:                increasing engine speed to a high level engine speed;        cutting fuel supply to said engine at said high level engine speed and registering said output signal from said at least one exhaust gas sensor at least a certain a time period after fuel supply cut; and        diagnosing NOx measurement performance of said at least one exhaust gas sensor on the basis of said registered sensor output.        
The second diagnosing sequence uses the moment of inertia of the engine that is available at a high engine speed to pump in air into the exhaust system by means of the combustion pistons. The pumped in air results in a very low NOx level within the exhaust system at the location of the exhaust gas sensor, thereby facilitating diagnosing of very low NOx emission level measurement performance by the exhaust gas sensor.
The first diagnosing sequence may preferably be performed before said second diagnosing sequence. This order of testing, where the fuel injection is cut such that the engine will automatically stop at the end of the second diagnosing sequence, has the advantage of allowing a natural and efficient termination of the self-diagnosing method.
The SCR system may comprise a SCR catalyst and a reductant injector, wherein said at least one exhaust gas sensor may be arranged downstream said SCR catalyst. The method may further comprise a third diagnosing sequence of:                controlling said reductant injector to perform in a non-reductant injection state and in a reductant injection state;        registering an output signal from said at least one exhaust gas sensor when said reductant injector performs in each of said non-reductant injection state and said reductant injection state; and        diagnosing NOx conversion efficiency of said SCR catalyst on the basis of said registered sensor output.        
By controlling the reductant injector to alternative at least once between a non-reductant injection state and in a reductant injection state, the NOx conversion efficiency of the SCR catalyst can be accurately diagnosed on the basis of the registered sensor output. If for example the reductant injector operates in a non-reductant injection state, and subsequently starts injecting reductant according to a injection model or the like, such that the reductant injector starts operating in a reductant injection state, the difference in measured NOx level by means of the exhaust gas sensor can be compared with a value provided by the diagnosing method, and based on this comparison a conclusion can be derived concerning the function and NOx conversion efficiency of the SCR catalyst.
The third diagnosing sequence may comprise repeating at least one shift between said non-reductant injection state and said reductant injection state, and registering said output signal from said at least one exhaust gas sensor at least before and after each of said repeated shifts. Repeated shifts may result in a more accurate diagnosis of the SCR conversion efficiency.
The first diagnosing sequence may advantageously be performed before said third diagnosing sequence, because this allows the self-diagnosing method to eliminate any erroneously determined NOx conversion efficiency due to erroneous exhaust gas sensor measurements. Consequently, in case the NOx conversion efficiency does not reach an expected level, the cause can be narrowed down to malfunctioning SCR catalyst, reductant injector failure, or bad reductant quality, or the like. The inventive method is consequently able to differ between exhaust gas sensor failure and SCR conversion failure.
The third diagnosing sequence may preferably be performed before said second diagnosing sequence. This order of testing, where the fuel injection is cut such that the engine will automatically stop at the end of the second diagnosing sequence, has the advantage of allowing a natural and efficient termination of the self-diagnosing method.
The exhaust gas sensor may preferably be sensitive also to oxygen, such that the exhaust gas sensor is capable of measuring the oxygen concentration of the exhaust gas for determining a lambda (λ) value of the air/fuel mixture entering the cylinders, wherein said method further comprising diagnosing oxygen measurement performance of the exhaust gas sensor on the basis of said registered sensor output. Diagnosis of both NOx detection performance and oxygen detection performance can thus be realised simultaneously, thereby facilitating an efficient self-diagnosing method. Oxygen measurement performance is very important for operating the engine with the correct air/fuel ratio, such that low NOx emission levels are generated.
The SCR system may further comprise a SCR catalyst and at least two exhaust gas sensors, wherein a first exhaust gas sensor of said at least two exhaust gas sensors is arranged downstream of said SCR catalyst, and a second exhaust gas sensor of said at least two exhaust gas sensors is arranged upstream of said SCR catalyst, wherein both said first and second exhaust gas sensors being sensitive to NOx gas. The first diagnosing sequence would then comprise the steps of:
registering an output signal of each of said first and second exhaust gas sensors when the engine performs in each of said high NOx output engine operating state and low NOx output engine operating state; and                diagnosing NOx measurement performance of said first and second exhaust gas sensors on the basis of said registered sensor output. By means of the two exhaust gas sensors, one positioned on each side of the SCR catalyst, a very accurate measurement of the NOx emission level entering and leaving the SCR catalyst may be determined, thereby facilitating an accurate diagnose of the SCR NOx conversion efficiency. Without a gas exhaust sensor upstream the SCR catalyst, the NOx emission level entering the SCR catalyst must be estimated, thereby reducing the reliability and accuracy of the self-diagnosing method.        
The method may include an initial step of controlling that an exhaust gas temperature of said SCR system exceeds a predetermined temperature level. Thereby any fluctuations in engine NOx emission values, and/or exhaust gas sensor measurement performance, and/or SCR conversion efficiency can be eliminated. Furthermore, the NOx catalyst requires that the temperature is above about 200° C. to fully operate in the third test sequence.
The SCR system may comprise a SCR catalyst and a reductant injector, wherein said at least one exhaust gas sensor is arranged downstream said SCR catalyst, wherein the method includes an initial step of controlling that that said reductant injection is stopped, and that a SCR catalyst ammonia storage level is substantially below the current maximal ammonia storage capacity of said SCR catalyst. It is advantageous, especially before entering the third test sequence, to empty the ammonia buffer of the SCR catalyst, because otherwise the stored ammonia in the SCR catalyst will uphold a relatively high NOx conversion efficiency also after stopping the reductant injection, thereby potentially leading to incorrect assumptions with respect to NOx conversion efficiency of the SCR system, and/or the reductant injection system, and/or the engine NOx emission level.
The method may comprise the initial step of activating said at least one exhaust gas sensor first after an exhaust gas temperature associated with said at least one exhaust gas sensor exceeds a predetermined temperature level. Avoiding manual activation of the sensor eliminates the risk that the sensor is activated at a too low temperature, which activation could potentially damage the sensor.
A computer program may also be provided, which program comprising program code means for performing all the steps of at least claim 1 when said program is run on a computer.
A computer program product may also be provided, which product comprising program code means stored on a computer readable medium for performing all the steps of at least claim 1 when said program product is run on a computer.
A computer system for implementing a method for diagnosing a selective catalytic reduction (SCR) system of a vehicle may also be provided. The vehicle comprises an internal combustion engine, wherein said SCR system is arranged downstream of the engine, wherein the SCR system comprises at least one exhaust gas sensor being sensitive to nitrogen oxides (NOx) gas, the computer system comprising a processor operable to:                ensuring that said vehicle is in a stationary state;        controlling said engine to operate in a high NOx output engine operating state and in a low NOx output engine operating state;        registering an output signal of said at least one exhaust gas sensor when the engine operates in each of said high NOx output engine operating state and low NOx output engine operating state; and        diagnosing NOx measurement performance of said at least one exhaust gas sensor on the basis of said registered sensor output.        